Originally Posted by The Guard
I thought it was fairly obvious that Blake's line was intended to show that Batman gave up the cape and cowl after the night Dent died. Nolan was trying to be subtle and not have Batman come right out and say "I last wore the cowl the night Dent died". Any "ambiguity" to that is pretty pointless, because the movie never shows or hints otherwise.
The whole point of the movie opening on Harvey Dent Day was that this was also the anniversary of Batman's last appearance. Full circle and all that.
Yup. Really straight forward.